The imaging devices generally comprise a set of pixels each having a semiconductor photosensitive region disposed under an optical filter integrated within the imaging device, for example a colored filter. Groups of pixels with red, green and blue filters may notably be used, for example in such a manner as to form Bayer patterns well known to those skilled in the art.
The optical filters designed to only let a single color through generally comprise a resin filter colored by pigments. These filters have the drawback of not being sufficiently robust and of not being sufficiently selective with respect to certain types of radiation, notably infrared radiation.
Alternatives to pigmented resins have therefore been provided.
Multilayer optical filters are also known, which are sometimes referred to as resonant filters, for filtering a type of radiation. These multilayer optical filters comprise a layer of a dielectric material generally disposed between two metal layers in order to form filters. The thickness and the optical characteristics of this dielectric layer notably determine the frequency of resonance of the resonator and they thus determine the wavelength of the photons that are able to reach a photosensitive region disposed under the filter. These multilayer filters may be filters designed to allow the visible colors or else infrared radiation to pass through. They may notably be disposed on top of imaging devices such as avalanche photodiodes triggered by a single photon (Single Photon Avalanche Diode).
Reference may be made to the published PCT Application WO 2008/012235 (incorporated by reference) which describes an imaging device comprising metal/dielectric multilayer filters formed on top of an interconnection part of an integrated circuit, over photosensitive regions. This interconnection part is commonly denoted by the acronym BEOL for “Back End Of Line”. The multilayer filters described in this document have the drawback of having too low a transmission in the neighborhood of the wavelength to be allowed through. These multilayer filters also have the drawback of not offering a high enough rejection of the wavelengths that it is desired to block.